1. Field of the Invention
The present invention relates to a grazing incidence mirror, a lithographic apparatus including such a grazing incidence mirror, a method for providing such a grazing incidence mirror and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which may be referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, for example around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
Optical systems like lithographic apparatus may include grazing incidence mirrors, which have a mirror surface layer of ruthenium. These grazing incidence mirrors may, however, lead to a large loss in reflectivity and of the desired radiation. Reflectivities at EUV wavelengths are already low compared to reflectors at longer wavelengths which is a particular problem since a typical EUV lithographic system may have several mirrors. For example, a EUV lithographic system may have nine mirrors: two in the illumination optics, six in the imaging optics plus the reflecting mask. It is therefore evident that even a small decrease of 1–2% in the peak reflectivity of a single mirror will cause a significant light throughput reduction in the optical system.